1. Field of the Invention
This invention relates to the fabrication of integrated circuits in general and, more particularly, to the fabrication of polysilicon on silicon integrated circuits.
2. Description of the Prior Art
Polycrystalline silicon, commonly known as polysilicon, is widely used in integrated circuit manufacturing. Polysilicon may be used as conductors and as contacts to the integrated circuit's silicon substrate as well as emitters in high performance bipolar integrated circuits. For example, in MOS integrated circuit technologies, the polysilicon may be used as contacts to source and drain diffusions in the silicon substrate of MOS transistors. In bipolar integrated circuit technology, using polysilicon for emitters results in the highest performance silicon bipolar transistors to date.
As linewidths shrink, the resistance of the contact between the polysilicon and the silicon substrate (or between polysilicon conductors) becomes more and more critical since less area is available for the contact. Similarly, to further improve the performance of bipolar transistors, the size of the emitter is reduced. As is the case for MOS devices, the smaller the emitter, the more critical the resistance of the interface (contact) between the polysilicon and the silicon substrate becomes.
Most prior art integrated circuit fabrication processes deposit the polysilicon in one apparatus after cleaning the silicon wafers in another. A typical wafer clean removes most of the contaminants on the silicon by an operator dipping the wafers in a dilute solution of hydrogen fluoride and then rinsing them in deionized water. This clean is done at room temperature and at normal atmospheric pressure. However, the surface of the silicon may not be sufficiently clean for a low resistance polysilicon to silicon contact to be made. Native oxides will form on the exposed silicon surfaces almost immediately when exposed to air after the wafers are cleaned and before they are placed in the polysilicon deposition reactor. In addition, other contaminants may adhere to the exposed silicon, forming defects at the silicon-to-polysilicon interface.
When polysilicon is deposited on the imperfectly cleaned silicon (or polysilicon) surface and the area of the interface between the polysilicon and silicon is sufficiently small, the resulting contact may not have low enough resistance to allow the integrated circuit to be operational. Even if the contact resistance is low enough for the circuit to be operational, the reliability of the circuit will be degraded since the native oxide "defect" could change and increase the contact resistance. Similarly, the junction between the polysilicon emitter and silicon substrate in a bipolar transistor may have defects therein caused by the native oxides. These defect areas may form recombinant zones which increases the leakage and series resistance of the junction while decreasing the emitter efficiency, thereby degrading the overall performance of the transistor.